The present invention relates to an optical apparatus for reproducing information recorded on an information recording medium, and more particularly, to a track transverse detection signal generating circuit.
Previously optical disks have served as optical information recording medium which use laser technology. As the optical disk, a laser video disk (an LVD) is well known for recording visual information, while a digital audio disk (a DAD) is well known for recording audio information, the DAD is based upon a compact disk (a CD) system.
The optical disk stores information recorded on its spiral track which extends from the inner portion toward the outer portion, the track being formed by a multiplicity of signal pits.
Optical disk players such as a LVD player and a DAD player are used to serve as an optical apparatus for reproducing recording information. The reproduction output is obtained by reading information from a surface of the optical disk on which information has been recorded, the optical disk player having an optical pickup therefor.
The optical pickup follows the track of the optical disk rotating at a predetermined rotational speed and applies an optical beam spot to the track so as to convert a beam reflected from a signal pit into an electric signal (an RF signal. RF: an abbreviation of Radio Frequency).
In order to cause the optical pickup to follow the signal track, it is necessary for the optical pickup to be slid in a radial direction of the optical disk and for the optical beam spot to be correctly applied to the track. A slide feeding mechanism and a tracking actuator are therefore used. A servo circuit is employed for the purpose of reliably controlling a slide feed distance and a tracking action. A specific servo circuit includes a slider servo circuit and a tracking servo circuit.
When the optical disk player reproduces information, the optical beam radiated from the optical pickup accurately traces the track from its inner portion toward the outer portion. However, in recent years optical disk players have been improved to offer a multiplicity of special functions such as a high speed searching function capable of conducting a retrieval of a required image and a still image reproducing function. When the above-described special operations are performed, a track jump is necessary during which the optical pickup jumps and slides over a plurality of tracks in the radial direction of the optical disk, (i.e., the optical pickup traverses the tracks).
In this case, the optical pickup detects a location in the optical disk during the jump by using the reflected light beam spot so as to supply a control signal to the tracking servo circuit and the slider servo circuit. The tracking servo circuit is therefore arranged to include a track transverse detection signal generating circuit. In the tracking servo circuit, before the pickup is moved to a target track, the number of tracks located between a current track and the target track is calculated. The pickup is then moved until the traversed number of tracks coincides with the calculated number of tracks.
In order to count the number of tracks, a zero-cross comparator is used for detecting a zero-crossing of a tracking error signal. A comparison output from the zero-cross comparator is generally used for a track transverse detection signal. In this case, a low pass filter (LPF) for determining a low cutoff frequency is provided on the input side of the zero-cross comparator to eliminate the influence of intermediate and high frequency component noises. The filter also eliminates the leakage of a digital voice signal in a video disk, whereby stability of track-jumping is maintained.
When the cutoff frequency is low and the pickup is moved quickly to perform high speed jumping, a gain sufficient to detect the zero-crossing of the tracking error signal cannot be obtained by the zero-crossing comparator due to delay characteristics of output signals of the low pass filter and a high speed track traversal of the pickup. Therefore, the track transverse detection signal cannot be accurately detected at the time of the high speed track jumping.